Article
Emergence of three-level superconductivity during annealing of YBCO + ε-Fe2O3
D. M. Gokhfeld
Kirensky Institute of Physics Krasnoyarsk Scientific Center, Siberian Branch, Russian Academy of Sciences, Akademgorodok St., 50/38, 660036, Krasnoyarsk, Russia
S. V. Semenov
Kirensky Institute of Physics Krasnoyarsk Scientific Center, Siberian Branch, Russian Academy of Sciences, Akademgorodok St., 50/38, 660036, Krasnoyarsk, Russia
M. I. Petrov
Kirensky Institute of Physics Krasnoyarsk Scientific Center, Siberian Branch, Russian Academy of Sciences, Akademgorodok St., 50/38, 660036, Krasnoyarsk, Russia
Abstract
Polycrystalline high-temperature superconductors typically exhibit two-level superconductivity originating from granule cores and intergranular boundaries. Three superconducting subsystems are discovered in high-temperature superconductor YBa2Cu3O7-δ (YBCO) doped with ε-Fe2O3 nanoparticles (average diameter 9 nm) and annealed at 930 °C for 12 hours. These subsystems have different superconducting transition temperatures and exhibit different responses to magnetic fields.
We investigated transport properties of a series of samples with ε-Fe2O3 weight fractions x from 0 to 0.0741. Temperature‑dependent electrical resistivity ρ(T) was measured using a standard four‑probe method in magnetic fields up to 0.01 T. For low doping levels (x ≤ 0.001), only two superconducting subsystems are observed. At x = 0.00299, the derivative dρ/dT reveals three distinct peaks corresponding to three transition temperatures: 92.8 K (granule cores), 90.3 K (intergranular boundaries), and 86 K (a new subsystem). This third subsystem originates from surface regions of YBCO granules where iron ions diffuse into the crystal lattice during prolonged annealing, locally suppressing the critical temperature. Unlike the intergranular boundaries, this surface subsystem shows weak sensitivity to magnetic fields below 0.01 T. Higher doping (x ≥ 0.00695) leads to a semiconductor‑like normal‑state resistivity and lack of the intergranular boundary transition in the superconducting state (above 77 K). Our findings demonstrate controlled creation of multi‑level superconducting structures via magnetic nanoparticle doping and subsequent annealing, and also are important for understanding pinning mechanisms in novel superconducting systems.
Keywords: high-temperature superconductors; YBCO; ε-Fe2O3; nanoparticles; three-level model; surface pinning.
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